
% CHAPTER 2
\chapter{BACKGROUND}
\label{chp:background}
This chapter will provide background information of the proposed solution. Brief information will be given for WiMAX and standardization activities. IEEE Std 802.16-2009 and IEEE Std 802.16j-2009 will be explained with details used in thesis. Overview of MobileIP protocol will be mentioned as the last topic of this chapter. 
\section{WiMAX Standards}
"WiMAX, meaning Worldwide Interoperability for Microwave Access, is a telecommunications technology that provides wireless transmission of data using a variety of transmission modes, from point-to-multipoint links to portable and fully mobile internet access"[4]. Nowadays WiMAX is also known as 4G of wireless communication technology. IEEE 802.16 standard is basis for the WiMAX. WiMAX Forum and IEEE 802.16 Working Group are official standardization organizations [4]. 802.16 working group composed of many task groups working on different aspects of wireless broadband standardization. List of current standards and their status are given in Table \ref{tab:IEEE80216ProjectsAndStandards}. 802.16d standard defines fixed broadband wireless access and it is first accepted standard. 802.16e introduces mobility and merged with 802.16d, both them are included in IEEE Std 802.16-2009.
\input{chapter2/table1}
\subsection{IEEE 802.16}
IEEE 802.16 standard defines fixed and mobile wireless broadband access. Last version of the standard released at 2009 which combines IEEE 802.16d and IEEE 802.16e. IEEE 802.16d Standard specifies the fixed wireless broadband access. Only PMP LOS communication was supported in this standard. 802.16e introduces the mobility and compared to its legacy IEEE 802.16d, has lower data rate. IEEE 802.16 standard specifies the MAC and PHY layers. MAC layer supports point-to-multipoint (PMP) architecture. Design of MAC supports different types of PHY layer specification [7]. OFDMA PHY is suitable physical layer for mobile WiMAX systems.
\subsubsection{IEEE 802.16 OFDMA PHY}
IEEE 802.16 OFDMA (Orthogonal Frequency Division Multiple Access) PHY operates in frequencies less than 11 GHz.  OFDM frequency division multiplexing scheme divides frequency band into many subcarriers. Data in each subcarrier is modulated traditionally as if sending data on parallel streams. OFDM supports single user at a time. OFDMA groups subset of subcarriers to form subchannels. By assigning these subchannels different subscribers, multiple users can communicate concurrently on same channel. Example OFMDA symbol shown in Figure \ref{fig:chapter2_0}. As shown in the Figure \ref{fig:chapter2_0}, subchannels may be constructed from inconsecutive subcarriers. In addition to OFDMA, IEEE 802.16 PHY support Adaptive Antenna Systems (AAS) and Multiple Input Multiple Output (MIMO) technology which enhances the capacity and coverage of network [8].
\input{chapter2/figure1}
Minimum data allocation unit of OFDMA PHY is a slot. Slot is two dimensional data structure with frequency and time domain. Time domain is composed of OFDMA symbols and frequency domain consists of subchannels. A Burst is a user data to be sent in one OFDMA frame. Since multiple users can communicate at the same time OFDMA frame consists of many bursts. When transmitting data, bursts should be mapped on slots in a frame. Example allocation of slots in OFDMA frame is shown in Figure \ref{fig:chapter2_1}. ODMA frame is divided into two main parts for Uplink (UL) and Downlink (DL) data. Each frame starts with preamble and ends with a guard. UL and DL parts are also separated with guard.
\input{chapter2/figure2}
\subsubsection{IEEE 802.16 MAC}
IEEE 802.16 MAC layer composed of three sublayers which are Service Specific Convergence Sublayer (CS), MAC Common Part Sublayer (MAC CPS) and Security Sublayer. Reference model is given on Figure \ref{fig:chapter2_2}. CS is adaptation layer for different upper layer protocols like ATM or TCP/IP. CPS is responsible for system access, connection management and bandwidth allocation.

MAC layer supports point-to-multipoint (PMP) architecture which means one or more mobile subscribers (MS) connect to one base station (BS). In downlink (DL) direction, BS transmits data without coordinating with subscribers. DL transmission is broadcast, every subscribers receives the transmitted frame. In DLMAP, sent in DL frame, it may not be explicitly stated which portion of subframe belongs to a specific subscriber. In such cases subscribers inspect the connection id (CID) of incoming packets and determine whether it belongs to itself. In uplink (UL) direction, users share uplink frame by adhering on a transport protocol. UL subframe allocation is determined by BS according to subscribers' demands. MAC layer allocates bandwidth according the transport connections' QoS class. UL frame sharing is handled by scheduling services implemented using unsolicited grants, contention and polling mechanisms [7].
\input{chapter2/figure3}
\textbf{Addressing and Connections}

Every 802.16 device has 48 bit MAC address which uniquely identifies the air interface. This address is used to establish the appropriate connections for SS. Connections in IEEE 802.16 are identified with 16 bit CIDs. During SS initialization three pairs of management connections are established which are basic connections (UL and DL), primary connections (UL and DL) and secondary connections (UL and DL). These connections are used for different level QoS management messages. Basic connections are used for urgent, short management messages. For longer and delay tolerant messages, primary connections are utilized. Secondary connections are dedicated to delay tolerant standard based messages like TFPT and DHCP.
\input{chapter2/figure4}
CIDs of connections should be assigned in the RNGRSP, REGRSP or MOB-BSHO-REQ/RSP messages. UL and DL connections of same type are assigned to same CID. For each QoS class bandwidth demand, a new transport connection is established.
\input{chapter2/table2}

\textbf{MAC PDU Formats and Management Messages}

IEEE 802.16 MAC PDU consists of three parts as shown in Figure \ref{fig:chapter2_3}. MAC header and CRC parts are fixed length and Payload is variable size. Some of the UL management messages may consist of just MAC header. OFDMA PHY layer requires CRC part included in MAC PDUs. Payload may contain zero or more subheaders.

MAC management messages starts with Type field and message payload follows it. Management messages carried on basic, primary, initial ranging and broadcast connections. Messages carried on basic and primary connection are not fragmented. Some of the MAC management messages listed in Table \ref{tab:MACManagementMessages} with their descriptions and connections.
\input{chapter2/table3}

\textbf{Scheduling}

For each transport connection, there is an associated scheduling service to handle the data. Scheduling mechanism differs according to service flow over transport connections. There are four scheduling types defined in MAC layer to meet bandwidth and delay requirements of different user demands. QoS classes and descriptions explained in Table \ref{tab:QoSClasses}. Requirements are parameterized in order to be used in scheduling algorithm. Algorithms employ polling, unsolicited grants and contention methods according to QoS parameters. For example for VoIP traffic, UGS QoS is selected and unsolicited grants given to subscriber to guarantee bandwidth for constant bitrate traffic.  Unsolicited grants method reserves bandwidth upon a demand from subscriber if enough resource is available. Polling methods are used for services that requires different amount of bandwidth during transmission. BS polls subscriber whether it should increase or decrease the bandwidth. Contention is used for services which are delay tolerant like FTP. Every subscriber tries to get as much as in contention period and all of them have equal chance.

\textbf{Network Entry and Initialization}

Network entry and initialization scenario for SS is described in activity diagram given in Figure \ref{fig:chapter2_4}.
\input{chapter2/figure5}
When a new SS enters to network, it starts to scan the wireless media and obtains DL-MAP and UL-MAP of neighbor BSs. Receiving a DL-MAP, SS is synchronized with BS. DL-MAP and messages describes the burst allocations of a frame for downlink and uplink respectively. SS prepares RNG-REQ for initialing ranging process after receiving DCD and UCD which give information about downlink and uplink channel respectively. Receiving RNG-RSP message ranging process is ends. SS sends SBC-REQ which contains information about SS capabilities and BS respond back with SBC-RSP which contains information about the BS's capabilities. By this way SS and BS negotiate with their common capabilities. SS sends REG-REQ the register BS after authorization phase passed. If BS accepts SS registration request, it responds with positive REG-RSP message. After this point if SS is accepted, then management and transport connections established and SS become operational. 

\textbf{Handover} 

Handover is process of a SS moves from radio interface of serving BS to radio interface of neighboring BS. Main reasons for handover are better signal quality and better QoS. When a SS moves, signal received from serving BS may attenuate. Interference and fading also affects the signal quality received by SS. In such cases taking service from other neighboring BS which has better signal, is more appropriate. Sometimes SS could not get enough service quality from the serving BS. Connecting to BS which has more available bandwidth is preferable.
\input{chapter2/figure6}
BSs know the topology of network by communicating with each other over backbone network. BS broadcast topology information periodically with MOB\_NBR-ADV message. MSs learn the neighboring BSs from this message. Also SS could requests scanning interval from BS to obtain neighboring BS information. MS obtains scanning interval by sending MOB\_SCN-REQ to serving BS.  Handover decision could be taken by either MS or serving BS. If MS decides, it sends MOB\_MSHO-REQ to serving BS. If vice versa, BS sends MOB\_BSHO-REQ to MS. After this point MS starts synchronize with target BS as in initialization phase. The rest of the process is similar to MS network entry and initialization as depicted in Figure \ref{fig:chapter2_5}.
\subsection{IEEE 802.16j Multihop Relay Specification}
\input{chapter2/figure7}
IEEE 802.16j standard is developed as an amendment to IEEE Std 802.16 by IEEE Multihop Relay (MR) task group. The standard updates and expands IEEE 802.16 for covering multihop relay operations. IEEE 802.16j describes data structures and control mechanisms for MAC common sublayer, security sublayer and physical layers of the MRs [1]. IEEE 802.16j standard designed to operate with legacy IEEE 802.16 mobile subscribers (MS). Introducing multihop relaying concept brings cost effective solutions for extending coverage and increasing the capacity. Specification supports mobility for relay stations (RS). Three types of relays exist according to deployment scenario, which are mobile, fixed and nomadic. Main use cases of RSs are shown in Figure \ref{fig:chapter2_6}.

IEEE 802.16j defines two types of operation modes for relays which are transparent and non-transparent mode. In transparent mode RSs do not send framing information which contains scheduling information for subscribers. Management messages should be received directly from the BS by subscribers in this mode. Non-transparent relays can send framing information, therefore management messages sent by RS to its subordinate subscribers.

There are two types of scheduling related to these operation modes: centralized scheduling and distributed scheduling. In centralized scheduling, BS is responsible for assigning UL and DL slots to each subscriber even if connected over RS. In distributed scheduling every RS generates DLMAP and ULMAP which contains the scheduling information [2]. 

\textbf{Transparent mode:} 

\input{chapter2/figure8}
Relays operating in transparent mode do not have transmits framing information.  Therefore centralized scheduling mode is used. Subscriber could connect to BS at most over two hops. Consequently relays operating in transparent modes do not extend the coverage of BS. These types of relays used for increasing the network capacity. Example usage is shown in Figure \ref{fig:chapter2_7}. 

\textbf{Non-transparent mode:}

\input{chapter2/figure9}
Relays in this mode enables extending coverage by sending framing information. Framing information can be generated by RS or can be directly forwarded according to selected scheduling mechanism [2]. Multihop topologies supported in both distributed and centralized scheduling. Due to interference caused by sending management messages by relays, capacity enhancement is not well as in transparent mode. Example usage of non-transparent relays is shown in Figure \ref{fig:chapter2_8}. Comparison of two modes is given in Table \ref{tab:ComparisonBtTransNontrans}.
\input{chapter2/table4} 
\subsubsection{PHY Layer Specification}
IEEE 802.16j provides some modifications to frame structure to capture the multihop relaying needs. IEEE 802.16 frame consist of UL and DL subframes. UL and DL subframes divided into two zones, one is for ordinary access zone which contains MS bursts and the other is relay zone for relay burst. UL and DL access zone structure is similar to legacy IEEE 802.16 UL and DL access zone. DL Relay zone starts R-MAP which contains framing information for UL and DL transfers, and relay fast recovery channel (R-FCH). The rest of DL zone is allocated for relay DL bursts. UL Relay zone contains relay UL bursts. UL and DL bursts contain data generated from or destined to relay nodes. The structure of an IEEE 802.16 PHY frame is shown in Figure \ref{fig:chapter2_9}.
\input{chapter2/figure10}
\subsubsection{MAC Layer Specification}
IEEE 802.16j MAC layer specification includes data structures and control mechanisms for multihop relaying. New management messages are introduced and some of the former messages modified for relay stations. Concerned management messages listed in Table \ref{tab:RelayMACMngPack}. 
\input{chapter2/table5}

\textbf{Addressing and Connections}

Connections defined in IEEE 802.16 standard are applicable for RS and MR-BS. Connections between MR-BS and SS may span over one or more RSs. Each RS must be associated with a BS in order to serve SSs and subordinate RSs. Connections are identified with CID as in former standard and CIDs must be unique in each cell. Multihop relay specification introduces tunnel connection concept. Tunnel connections are established between RS and MR-BS or superordinate RS. MAC PDUs from different connections may use same tunnel connection. Using tunnel connection, end-to-end transport connections passing through the intermediate links decreased to one. Usage of tunnel connections is optional.

Paths between MR-BS and SS can be established either by using tunnel connections or CID based connections. In CID based connection, R-links with same CID are established between RS and its superordinate station. When a PDU arrived to RS, packet forwarded to connection with CID stated in the MAC header. Figure \ref{fig:chapter2_10} illustrates the CID based connections. 
\input{chapter2/figure11}

\textbf{Relay Path Management and Routing}

Based on topology, MR-BS determines the path between itself and access station of MS. Path selection decision is made according to tree topology constraints and available resources. In tree topology a RS should connect to one superordinate station. Path selection algorithm is not defined in IEEE 802.16j standard, it is left to vendors.

MR-BS selects access station of RS during network entry. RS performs measurement report to inform MR-BS about neighbors as shown in Figure \ref{fig:chapter2_12} and wait for RS\_AccessRS-REQ message. When MR-BS receives measurement report from RS via RS\_NBR\_MEAS-REP message, it performs path selection for RS as shown in Figure \ref{fig:chapter2_13}. If the access station for RS is changed MR-BS sends RS\_AccessRS-REQ message. Upon receiving the message RS sends MR\_Generic-ACK to MR-BS and initiate network reentry procedure as shown in Figure \ref{fig:chapter2_14}.
\input{chapter2/figure12}
\input{chapter2/figure13}
\input{chapter2/figure14}
There are two types of path management. One of them is embedded path management where there is no need to keep routing table. Second is explicit path management which requires routing table to keep the association between path and CIDs. In embedded path management CIDs assigned systematically so that arrived MAC PDU's next R-link is determined according to CID. MR-BS assigns CID blocks to its subordinate RSs and each RS also assigns consecutive block of CIDs to its subordinates. Example assignment is shown in Figure \ref{fig:chapter2_15}. According to the assignment in Figure 15, when it is needed to establish a transport connection between MR-BS and a MS connected to RS tagged with 'H', CID of the connection is picked between 1 and 100. If a MAC PDU with CID=9, arrives to intermediate RS B then RS will forward the PDU to R-link to RS D. RS D then forwards it to RS H.
\input{chapter2/figure15}
MR-BS detects the topology changes when a new MS or RS connect or disconnects from the network. Upon such topology changes, MR-BS rearranges the CID and PATH mapping. When a new MS or RS is attached to MR-BS, path for that subordinate station is determined by MR-BS and all the intermediate RSs are informed about the PATH-CID mapping with DSA-REQ message. Furthermore, when MR-BS decides to remove a path, it sends a DSD-REQ message to all intermediate RSs.

When a new connection is established, PATH-CID mapping in routing table should be updated. After MR-BS decides the path for routing the new connection, it sends path information which contains PATH ID and CID, to all intermediate RSs on the path. Upon receiving DSA-REQ message, RS insert entry to routing table and forwards message to subordinate RS which is the next hop on the path. A DSA-REQ message propagates until it reaches the access station. Access station responds with DSA-RSP message to inform all superordinate RSs and MR-BS about success of path creation. For removal of a path, MR-BS sends DSD-REQ messages to RSs on the path, and all the intermediate RS updates the routing table by removing associated the path entry.
\section{Mobile IP}
MobileIP is extension to IP protocol developed by IETF (Internet Engineering Task Force) to handle mobility issues. MobileIP enables mobile devices stay connected without changing home address while roaming. Mobile nodes which support Mobile IP have two IP addresses which are Home Address and Care Of Address. Home Address is assigned by Home Agent which runs at mobile nodes home network. Care Of Address is assigned when mobile node moves to another network. Care Of Address is temporary whereas home address is permanent. Care of address is not transparent to upper layer protocols. All upper layer protocols and applications use home address for addressing source and destination nodes.
\subsection{Overview of Protocol}
There are two types of agent defined in protocol. Home Agent is running on home network of the mobile node. Home agent maintains the mobility binding table which contains the home address and care of address mapping of mobile node. Foreign agent is running on the foreign network where the node is visiting temporarily, and gives service. Both services periodically broadcast the agent information in order to be discovered by mobile nodes.

When mobile node leaves the home network and enters to foreign network, it sends a registration request with lifetime information to foreign agent. Foreign agent resolves the home network of node by inspecting its home address, and sends a request with care of address to be assigned to node. Upon receiving the request from foreign agent, home agent keeps a record of mobile nodes current location and its care of address and then responds to request. After receiving response from home agent, foreign agent accepts connection request of visiting mobile node and updates the visiting lists. Registration process is summarized in Figure \ref{fig:chapter2_16}.
\input{chapter2/figure16}
After registration a tunnel between home agent and foreign is established to forward packets destined to home address of mobile node its current address. Since upper layers transparent to location change, IP packets are constructed with source address is home address. Packets are routed to destination as usual. Destination node of the traffic never realizes the location of the mobile node. If the correspondent node intends to send packet to mobile node, it uses home address as destination address. Routers will forward the packet to home network of the mobile node.  Receiving a packet destined to mobile node, home agent checks the mobility binding table and forwards the packet to care of address of the node by encapsulating packet with a new IP header. Encapsulated packet is sent to foreign agent over tunnel established in registration phase. Upon receiving encapsulated packet, foreign agent extracts original IP packet and sends it to mobile node. Tunneling operation is shown in Figure \ref{fig:chapter2_17}.
\input{chapter2/figure17}  